The present invention relates to a rolling bearing unit and constant velocity joint for wheels, specifically a rolling bearing unit and constant velocity joint for rotatably supporting a front wheel of the automobile of the front engine front wheel drive (FF) type or four wheel drive (4WD) with respect to the suspension.
In order to rotatably support a vehicle wheel or road wheel with respect to the suspension, various kinds of rolling-bearing units for wheels have been used wherein outer and inner races or rings can rotate freely by way of rolling members. Here, when a rolling-bearing unit for supporting the front wheel of the FF automobile or 4WD automobile, which is the driven wheel as well as the steering wheel, is combined with a constant velocity joint, it is necessary that rotation of the drive shaft be transmitted smoothly (maintaining uniform speed) to the vehicle wheel regardless of the steering angle applied to the vehicle wheel.
Also, when a rolling-bearing unit for supporting the driven wheel of the automobile with the independent type suspension is combined with a constant velocity joint, it is necessary that rotation of the drive shaft be transmitted smoothly (maintaining uniform speed) to the vehicle wheel regardless of the relative displacement between the differential gear and the driven wheel and of the steering angle applied to the vehicle wheel.
WO 97/24538 disclosed an eight ball constant velocity joint wherein (1) the ratio of the pitch circle diameter of the torque transmitting balls to the diameter of the torque transmitting balls is between 3.3 to 5.0, and (2) the ratio of the outer diameter of the outer joint member to the pitch circle diameter of the tooth profile of the inner joint member is set within the range from 2.5 to 3.5.
However, U.S. Pat. No. 5,221,233 had disclosed prior to WO 97/24538 an eight ball constant velocity joint, wherein determining the dimension relation of (1) and (2) above is an inherent setting matter under the limited conditions, which can be practically set by a person skilled in the art in the most optimum values taking into consideration the strength of the outer joint member, the strength of the inner joint member, and the strength of the cage.
In addition, in the eight ball constant velocity joint of WO 97/24538, (3) the cage has short and long pockets, the short pockets being arranged with a space of 180 degrees between them.
However, U.S. Pat. No. 5,509,856 (filed on Oct. 13, 1993, its German counterpart is published on Mar. 24, 1994) disclosed a technology of the six ball constant velocity joint where the width of the posts (webs or column sections) of the cage is sufficiently kept while being capable of incorporating the balls in it.
Specifically, since the balls already incorporated move circumferentially in the cage pockets when the cage is tilted to incorporate the remained balls, the pockets must be longer by that amount.
However, the last two balls for incorporation to be spaced apart from each other with a space of 180 degrees is not required to move circumferentially, so that the pockets spaced apart 180 degrees for the last two balls can be shorter. Therefore, the width of the posts (webs or column sections) of the cage can be wider to improve the cage strength.
On the other hand, U.S. Pat. No. 5,221,233 discloses a technology to make the number of balls eight to increase the load capacity. Upon combining these technologies, the former technology to make the incorporation convenient has no relation with the latter technology to increase the load capacity. Accordingly, there is no relation between the arrangement of shorter pockets spaced apart 180 degrees and the increase of balls from six to eight, and there is no special technical meaning in combining such technologies.
A relatively compact and lightweight rolling-bearing unit for wheels used with this kind of constant velocity joint has been disclosed in Japanese Patent Publication TokuKai-Hei No.7-317754.
FIG. 1 shows the construction of the bearing described in this disclosure. When installed in the vehicle or automobile, the outer race or ring 11, which does not rotate when supported by the suspension, has a first installation flange 12 for supporting it to the suspension around its outer peripheral surface and a double row of outer-nag raceways 13 around its inner peripheral surface. Disposed on the inside of the outer race or ring 11 is a hub 16 which comprises a first inner race or ring 14 and a second inner ring 15.
The first inner ring 14 has a second installation flange 17 for supporting the vehicle wheel on one end portion (left end portion in FIG. 1) and a first inner ring raceway 18 on the other end portion (right end portion in FIG. 1), both of which are cylindrical shaped.
The second inner ring 15 has a cylindrical section 19 on one end portion (left end in FIG. 1) for fitting the first inner ring 14 thereon, a housing portion 52, which is the outer member or ring of the constant velocity joint 1 (detailed later), on the other end portion (right end in FIG. 1), and a second inner ring raceway 20 around the outer peripheral surface in the middle portion. By placing several rolling members 21 between the outer-ring raceways 13 and the first and second inner-ring raceways 18, 20, respectively, the hub 5 can rotate freely inside the outer ring 11.
Moreover, a first attachment groove 22 is formed in the inner peripheral surface of the first inner ring 14 and a second attachment grooves 23 is formed in the outer peripheral surface of the second inner ring 15, where the first and second attachment grooves 22, 23 come together in alignment, and by placing a retaining ring 24 in both of these attachment grooves 22, 23, the first inner ring 14 is prevented from coming out of the second inner ring 15.
Furthermore, the outer peripheral edge of one end face (left end face in FIG. 1) of the second inner ring 15 is joined to the inner peripheral edge of a stepped section 25 formed around the inner peripheral surface of the first inner ring 14 by a weld 26, such that the first ring 14 and second inner ring 15 are joined together.
Also, a substantially cylindrical shaped cover 27a and a circular ring shaped seal ring 28a are located between the opening portion at one end of the outer ring 11 and the corresponding outer peripheral surface portion in the middle portion of the hub 16 while a substantially cylindrical shaped cover 27b and a circular ring shaped seal ring 28b are located between the opening portion on the other end of the outer a ring 11 and the corresponding outer peripheral surface portion in the middle portion of the hub 16. The substantially cylindrical shaped covers 27a, 27b are made of metal such as stainless steel, and the circular ring shaped seal rings 28a, 28b are made of elastic material such as rubber or elastomer.
These covers 27a, 27b and seals 28a, 28b seal of the area where the rolling members 21 are located from the outside, which prevents the grease in this area from leaking out, and also rain water, dirt or other foreign matter from getting into this area.
Moreover, there is a partition plate 29 inside of the middle portion of the second inner ring 15 to close off the inside of the second inner ring 15, which maintains the rigidity of the second inner ring 15, and prevents foreign matter that has gotten into the inside of the second inner ring 15 through the opening on the one end (left end in FIG. 1) from reaching the constant velocity joint 1 located on the inside of the housing portion 52.
The constant velocity joint 1 comprises an inner member or ring 2, retainer or cage 9 and multiple balls (engagement balls) 4 in addition to the housing portion 52. Of these components, the inner ring 2 is attached to the end of the drive shaft (not shown in the figures) which is driven and rotated by the engine by way of the transmission.
There are six inner engagement grooves 7 having an arc-shaped cross-section which are formed on the outer peripheral surface of the inner ring 2 such that they are separated at equal intervals in the circumferential direction and are perpendicular to the circumferential direction.
Moreover, there are six outer engagement grooves 8 having an arc-shaped cross-section which are formed on the inner peripheral surface of the housing 52, such that they are perpendicular to the circumferential direction. The outer engagement grooves 8 face the inner engagement grooves 7 for engagement, respectively.
The retainer 9 has an arc-shaped cross section and is entirely circular ring shaped and is placed between the outer peripheral surface of the inner ring 2 and the inner peripheral surface of the housing portion 52. At six places around the circumference of his retainer 9, pockets 10 are located at the position in alignment with the inner engagement grooves 7 and outer engagement grooves 8, and there is one ball 4 held inside each one of these pockets for a total of six balls 4. With each ball 4 held in a pocket 10, they can rotate freely along the inner engagement grooves 7 and outer engagement grooves 8.
When installing the rolling-bearing unit for wheels, that is constructed as described above, into a vehicle, the outer ring 11 is supported by the suspension through the first installation flange 12, and the vehicle wheel, which is a driven wheel or front wheel in this case is fastened to the first inner ring 14 by the second installation flange 17.
Also, the tip end portion of the drive shaft (not shown in the figures) which is rotated and driven by the engine by way of the transmission, fits inside the inner ring 2 of the constant velocity joint 1 by a spline engagement. When the vehicle is moving, the rotation of the inner ring 2 is transmitted by way of the multiple balls 4 to the hub 16 which includes the second inner ring 15, thereby rotating and driving the driven wheel or front wheel.
In the case of the conventional rolling-bearing unit for wheels as shown in FIG. 1, it is very difficult to made the unit more compact and lightweight. The reasons for this are as follows. The number of the inner engagement grooves 7, outer engagement grooves 8 and balls 4 located inside the constant velocity joint 1 which is built into the rolling-bearing unit described above is normally six for each.
Therefore, in the case of this kind of rolling-bearing unit with constant velocity joint 1 integrally built in, it is necessary to increase somewhat the outer diameter of the balls 4 of the constant velocity joint 1 in order that the required torque may be transmitted while maintaining the rolling fatigue life of the interfacing groves 7, 8 and the rolling surfaces of balls 4 in the constant velocity joint 1.
Accordingly, in the case of this conventional rolling-bearing unit for wheels, the diameter of the circumscribed circle of the outer engagement grooves 8, that is determined by the size of the diameter of the circumscribed circle of the balls 4, is larger than the diameter of the second inner ring raceway 20 around the outer peripheral surface of the second inner ring 15, which is one of the inner-ring raceways 18, 20 that are formed in rows around the outer peripheral surface in the middle portion of the hub 16.
Therefore, this conventional rolling-bearing unit for wheels comprises a rolling-bearing portion and a constant velocity joint portion which are lined up in series with each other in the axial direction as shown in FIG. 1, wherein the rolling-bearing portion is made up of the outer ring 11 with the outer-ring raceways 13 foamed in rows around the inner peripheral surface of the outer ring 11, the middle portion of the hub 16 with the first and second inner ring raceways 18, 20 formed around the outer peripheral surface in the middle portion of the hub 16, and the multiple rolling members 21 that are located between the first and second inner-ring raceways 18, 19 and the outer-ring raceways 13.
With the rolling-bearing portion and constant velocity joint portion arranged in series, the dimensions in the axial direction of the entire rolling-bearing unit for wheels become large, which also causes the weight to increase. The increased weight of the rolling-bearing unit would cause a larger unspringing load on the vehicle, and therefore it is desired that the rolling-bearing unit be made more compact and light weight.
It is possible to overlap the second inner-ring raceway 20 and part of the outer engagement grooves 8 in the radial direction by making the diameter of the second inner-ring raceway 20 larger than the diameter of the circumscribed circle of each outer engagement groove 8, so that the dimensions in the axial direction of the rolling-bearing unit for wheels is reduced.
However, if the diameter of the second inner-ring raceway 20 is simply enlarged, instead of being able to reduce the dimensions in the axial direction of the rolling-bearing unit for wheels, the dimensions in the radial direction become larger, causing the weight of the entire unit to increase by that amount, and thus making it impossible to achieve the more compact and light weight rolling-bearing unit.
A constant velocity joint is located between the automobile transmission and the drive wheels which are supported by an independent suspended-type suspension and regardless of the relative displacement between the differential gear and the drive wheel, or the steering angle applied to the drive wheels, the drive force from the engine is freely transmitted to the drive wheels with the same angular velocity all the way around.
The constant velocity joint used in this area has been disclosed previously, for example in Japanese Utility Model Publication JITSUKAI Nos. Sho 57-145824xcx9c5, Sho 59-185425, and Sho 62-120-21.
This prior constant velocity joint 1, as shown in FIGS. 2 thru 4, transmits a rotating force by way of six balls 4 located between an inner member or ring 2 and outer member or ring 3. The inner ring 2 is fixed to the outer end of a shaft 5, which is rotated and driven by the transmission.
Therefore, the inner ring 2 is formed at its center portion a spline hole 51 for engagement with a male spline portion provided on the end portion of the shaft 5.
Also, the outer ring 3 is fixed to the inner end of another shaft 6 which connects to the drive wheels. Six inner engagement grooves 7 arc-shaped in cross section are formed around the outer peripheral surface 2a of the inner ring 2, and are formed so that they are equally spaced in the circumferential direction and so that they are orthogonal to the circumferential direction. Moreover, six outer engagement grooves 8 arc-shaped in cross section are formed around the inner peripheral surface 3a of the outer ring 3, and are such that they face the inner engagement grooves 7 and are orthogonal to the circumferential direction.
Furthermore, a generally ring-shaped retainer 9 with arch-shaped cross section is fitted between the outer peripheral surface 2a of the inner ring 2 and the inner peripheral surface 3a of the outer ring 3. Six pockets 10 are formed at six locations in the circumferential direction around this retainer 9 such that they match the locations of the inner and outer engagement grooves 7, 8, and a ball (engagement ball) 4 is held inside each of these pockets 10 for a total of six balls 4. These balls 4 are held inside the pockets 10, so that they can freely roll along the inner and outer engagement grooves 7, 8.
The pockets 10, as shown in FIG. 3, are rectangular in the circumferential direction, and as the crossing angle xcex1 of the shaft, to be described later, changes, the space between balls 4 adjacent to each other in the circumferential direction can absorb this change in other words, the relationship between the bottom surfaces 7a of a pair of inner engagement grooves 7, and the relationship between the bottom surfaces 8a of a pair of outer engagement grooves 8, as shown by the dashed line in FIG. 5, is as the lines of longitude around the globe.
When the center axis of the inner ring 2 matches the center axis of the outer ring 3 (shaft crossing angle xcex1=180 degrees), the balls 4 are located, as shown by the double dashed line in FIG. 5, near what would correspond to the equator of the globe.
On the other hand, when the center axis of the inner ring 2 does not match the center axis of the outer ring 3 (shaft crossing angle xcex1 less than 180 deg.), as the constant velocity joint 1 makes one turn, the balls 4 make one cycle in the up and down direction shown in FIG. 5 (move alternately in the direction of the north pole and south pole of the globe) As a result, the space between a pair of balls 4 which are adjacent to each other in the circumferential direction is enlarged and shrunk, therefore the pockets 10 are rectangular shaped in the circumferential direction so that the spacing between the adjacent balls 4 can be enlarged and shrunk.
The bottom surfaces 7a of the inner engagement grooves 7 and the bottom surfaces 8a of the outer engagement grooves 8, are not concentric, as will be made evident in the aforementioned explanation. Moreover, The lines which correspond to longitudes are shifted a little for each set of engagement grooves 7, 8.
Furthermore, as shown in FIG. 2, regardless of the displacement of the one shaft 5 and the other shaft 6, the balls 4 are located in an equally divided surface Z, that is made by equally dividing the shaft crossing angle xcex1 of the shafts 5, 6, or in other words that is made by dividing the angle xcex1 that is formed by the center axis X of the one shaft 5 and center axis Y of the other shaft 6 which cross at point O. Therefore, the bottom surface 7a of inner engagement groove 7 is located above center axis X on the spherical surface whose center is point D that is separated a distance H from the intersection O, and the bottom 8a of outer engagement groove 8 is located above center axis Y on a spherical surface whose center is point E that is separated a distance H from the intersection O. However, the outer peripheral surface 2a of the inner ring 2, the inner peripheral surface 3a of the outer ring 3, and the inner and outer peripheral surfaces of the retainer or cage 9 are located on a spherical surface whose center is the intersection O, and sliding movement of the outer peripheral surface 2a of the inner ring 2 and the inner peripheral surface of the retainer 9, and the sliding movement of the inner peripheral surface 3a of the outer ring 3 and the outer peripheral surface of the retainer 9 is free.
In the case of the constant velocity joint 1 which is constructed as described above, when the inner ring 2 is rotated by the shaft 5, this rotation is transmitted to the outer ring 3 by way of the six balls 4, and rotates the shaft 6. If the positional relationship (shaft crossing angle xcex1) of the shafts 5, 6 changes, the balls 4 rotate along the inner and outer engagement grooves 7, 8 to allow for the displacement of the one shaft 5 and the other shaft 6.
The basic construction and action of the constant velocity joint was described above, however, recently combining this kind of constant velocity joint with a roller-bearing unit for wheels, where the wheel is rotatably supported by a suspension, is being studied. In other words, in order for the suspension to rotatably support a vehicle or road wheel, a rolling-bearing unit for wheels which rotates freely outer ring and inner ring through rolling members is used. By integrating this kid of rolling-bearing unit for wheels and the constant velocity joint, described above, it is possible to make the rolling-bearing unit for wheels and constant velocity joint more compact and light weight. This kind of integrated rolling-bearing unit for wheels and constant velocity joint is called a fourth-generation hub unit and has been previously disclosed in Japanese Patent Publication KOKAI No. Hei 7-317754 as previously mentioned on FIG. 1.
Explanation is repeated hereinafter. FIG. 1 shows the construction of this previously disclosed rolling-bearing unit. With the unit installed in an automobile, the outer ring 11 is supported by the suspension and does not rotate, and comprises an outer peripheral surface formed with a first installation flange 12 which is supported by the suspension, and an inner peripheral surface formed with multiple outer-ring raceway 13. There is a hub 16 located on the inside of the outer ring 11 which comprises first and second inner-ring members 14, 15.
Of these, the first inner-ring member 14 is cylindrical shaped and has a second installation flange 17 for supporting the road wheel located on the outer peripheral surface on one end side (left end side in FIG. 1), and a first inner-ring raceway 18 located on the other end side (right end side in FIG. 1).
On the other hand, the second inner-ring member 15 has a cylindrical section 19 on one end portion (left end portion in FIG. 1) which fits over and attaches to the first inner-ring member 14, and on the other end portion (right end portion in FIG. 1) there is a second inner-ring raceway 20 located around the outer peripheral surface in the middle and serves as the outer ring 3 of the constant velocity joint 1. Also, by placing several rotating members 21 between the outer-ring raceways 13 and the first and second inner-ring raceways 18, 20, it is possible for the hub 16 to rotate freely inside of the outer ring 11.
In the locations in alignment on where the inner peripheral surface of the first inner-ring member 14 meets and the outer peripheral surface of the second inner-ring member 15, there are attachment grooves 22, and by placing a attachment ring 24 over these attachment grooves 22, the first inner-ring member 14 is prevented from coming out of the second inner-ring member 15. Furthermore, the first and second inner-ring members 14, 15 are connected together by a weld 26 between the outer peripheral edge on one end face (left end face in FIG. 1) of the second inner-ring member 15, and the inner peripheral edge on a stepped section 25 that is formed around the inner peripheral surface of the first inner-ring member 14.
Furthermore, substantially cylindrical shaped covers 27a, 27b, made of metal such as stainless steel, and ring-shaped seal rings 28a, 28b, made of rubber or elastomer, are located between the openings on both ends of the outer ring 1 and the outer peripheral surface in the middle portion of the hub 16. These covers 27a, 27b, and seal rings 28a, 28b, cut off from the outside the area where the rolling members 21 are located, so as to prevent the grease in this area from leaking out and to prevent rain water or other matter such as dust from getting in. Moreover, a dividing plate 29 is located on the inside in the middle portion of the second inner-ring member 15 so that it covers the inside of this second inner-ring member 15, so as to securely maintain the rigidity of the second inner-ring member 15, and to prevent objects, which have gotten into the inside of this second inner-ring member 15 from the opening on the end (left end in FIG. 1) of this second inner-ring member 15, from getting into the area of the constant velocity joint 1a. 
When installing the rolling-bearing unit for wheels, which is constructed as described above, into an automobile or vehicle; the outer ring 11 is supported by the suspension by the first installation flange 12, and the vehicle wheel, which is the driven wheel, is attached to the first inner-ring member 14 by the second installation flange 17. Also, the tip end portion of the drive shaft, which is not shown in the figures and which is rotated and driven by the engine by way of the transmission, is connected to the inside of the inner ring 2 of the constant velocity joint 1 with splines. As the automobile moves, the rotation of this inner ring 2 is transmitted to the hub 16, which includes the second inner-ring member 15, by way of the several balls 4, and thus rotates and drives the road wheel.
In order to make the fourth-generation hub unit described above even more compact, it is effective to reduce the diameter of the circumscribed circle of the balls 4 of the constant velocity joint 1. Also, in order to decrease the diameter of this circumscribed circle, it is necessary to decrease the diameter of the balls 4, and in order to maintain the torque that can be transmitted by the constant velocity joint 1, it is necessary to increase the number of balls 4. Moreover, even if the number of balls 4 is increased due to these circumstances, in order to maintain the durability of the retainer or cage 9 which holds these balls 4, it is necessary to maintain the dimension in the circumferential direction of the column sections 30 (see FIGS. 2, 3, 6 and 9) which are located between pairs of pockets 10 in the retainer 9. This is because, if the length in the circumferential direction of these column sections 30 is insufficient, the strength of the retainer 9 will not be enough, and when used over a long period of time, damage such as cracking around the edges of the pockets 10 could occur. However, increasing the length of the column sections 30 is limited by the fact that interference to the balls 4 must be prevented.
In other words first, if the length in the circumferential direction of the pockets 10 must be large enough such that it is possible for the balls 4 to be displaced in the circumferential direction of the retainer 9 when rotated at the joint angle of the constant velocity joint 1 (angle when the positional relationship of the center axis of inner ring 2 and center axis of outer ring 3 shifts from the straight line state; supplementary angle of the shaft crossing angle xcex1 shown in FIG. 1). Second, the length must be large enough, so that the balls 4 can be installed in the pockets 10 of the retainer 9 after the inner ring 2, outer ring 3 and retainer 9 of the constant velocity joint 1 have been assembled.
In considering these points, a constant velocity joint 1 which uses more than six balls 4 and a column section whose length in the circumferential direction has been enlarged has been disclosed in Japanese Publication TOKUKAI No. Hei 9-177814, and is shown in FIGS. 6 thru 9. The constant velocity joint 1 of this disclosure is constructed so as to transmit rotational power between the inner ring 2 and outer ring 3 by way of eight balls 4. Also, in the construction of this disclosure, at eight places in the circumferential direction around the retainer 9 there are pockets 10a which are long in the circumferential direction and pockets 10b which are short in the circumferential direction, and they are alternately located such that they are evenly spaced (separating pitch angle is the same). Of these two kinds of pockets 10a, 10b, the short pockets 10b are large enough that neither edge in the direction of length of the pockets 10b interfere with the rolling surface of the balls 4 that are held inside the pockets 10b, even when the constant velocity joint 1 is used with a maximum joint angle. On the other hand, the long pockets 10a are large enough that neither edges of the pockets 10a in the direction of length interfere with the balls 4 installed already in the pockets 10a, even when the center axis of the inner ring 2 and the center axis of the outer ring 3 are inclined such that they exceed the maximum operating joint angle in order that the balls 4 are assembled in the pockets 10b. 
With the constant velocity joint disclosed in Japanese Publication TOKUKAI No. Hei 9-177814 and constructed as described above, by placing the balls 4 in the short pockets 10b after placing the balls 4 in the long pockets 10a, the balls 4 can be placed inside all of the pockets 10a, 10b. In other words, when placing the balls 4 inside these pockets 10a, 10b, the center axis of the inner ring 2 and the center axis of the outer ring 3 are inclined so as to exceed the maximum operating joint angle as shown in FIG. 9. When placing the balls 4 into the long pockets 10a, the edges of these pockets 10a and the edges of the inner engagement grooves 7 formed around the outer peripheral surface of the inner ring 2 match for one ball 4 or more. Therefore, the balls 4 can be properly placed inside these pockets 10a. Next, by tilting the center axis of the inner ring 2 and the center axis of the outer ring 3 as shown in FIG. 9, so that the balls 4 are placed in the short pockets 10b, the balls 4 already placed in the long pockets 10a, move inside the pockets 10a in the direction toward the short pockets 10b as shown by the dashed arrow in FIG. 8. Also, the centers of the short pockets 10b match the inner engagement grooves 7 formed around he outer peripheral surface of the inner ring 2 in alignment. Therefore, the balls 4 can be properly placed inside these pockets 10b. 
The state with the balls 4 placed inside the pockets 10a, 10b will be explained using FIG. 10. FIG. 10 diagrammatically shows the locations of the pockets 10a, 10b in the retainer 9 of the constant velocity joint and their respective lengths. There are four pockets of each kind 10a, 10b for a total of eight, and they are arranged so that they are equally spaced every 45 degrees (xcfx80/4 radian) around the circumference. The arc-shaped sections that are indicated with code numbers 1 thru 8 together with being shaded, show the positions and lengths of each of the pockets 10a, 10b. In other words, the center in the circumferential direction of each of these arc-shaped sections corresponds to the center in the direction of length of the pockets 10a, 10b. Moreover, the lengths of these arc-shaped sections indicate the amount that the balls 4 in the pockets 10a, 10b (see FIGS. 6 thru 9) move in the circumferential direction, corresponding to the lengths of the pockets 10a, 10b. In other words, the balls 4 that are placed in the long pockets 10a freely move a distance xcex30 in the circumferential direction on either side of the center position. On the other hand, the balls 4 that are placed in the short pockets freely move a distance xcex31 in the circumferential direction on either side of the center position. These angles xcex30 and xcex31, that are shown in FIG. 10 and also in FIG. 22 to be described later, have been exaggerated for the purpose of clarity. Moreover, the work of placing the balls 4 in different arc-shaped sections which are concentrically placed and indicated with the code numbers {circle around (1)} thru {circle around (8)}, is performed in order in the radial direction from the inside out Placing the balls 4 in the same arc-shaped sections is not performed at the same time, but which is placed first does not matter.
When placing the balls 4 in the retainer 9 disclosed above, first, balls 4 are place in order one at a time in the long pockets 10a which are indicated by code numbers {circle around (1)}, {circle around (3)}, {circle around (5)} and {circle around (7)}. Next, balls 4 are placed in order one at a time in the four short pockets 10b that are indicated by code numbers {circle around (2)}, {circle around (4)}, {circle around (6)} and {circle around (8)}. In order to be able to perform this work, the center axis of the inner ring 2 and the center axis of the outer ring 3 are tilted as shown in FIG. 9, and as is shown by the arrows in FIG., 10, the balls 4 that are already placed in the long pockets 10a move in the direction toward the short pockets 10b. However, since the lengths of these pockets 10a are long, the inner surface of the lengthwise edges of the pockets 10a do not interfere with the rolling surfaces of the balls installed already in the pocket 10a, before the short pockets 10b line up with the inner engagement grooves 7 formed on the outer peripheral surface of the inner ring 2 for one or more balls 4. Therefore, it is possible to greatly tilt the center axis of the inner ring 2 and the center axis of the outer ring 3 and to properly place the balls 4 in the short pockets 10b. 
In the case of the constant velocity joint stat is disclosed in Japanese Patent Publication TOKUKAI No. Hei 9-177814, two kinds of pockets 10a, 10b with differing length in the circumferential direction are equally spaced and alternated in the circumferential direction. Therefore, in comparison with a constant velocity joint that uses only one kind of pocket, it is possible to increase the length in the circumferential direction of the column section located between adjacent pockets, however, they still cannot be made sufficiently large.
In other words, in the prior construction as disclosed in the aforementioned disclosure, there are long and short retainer pockets 10a, 10b, however when placing the balls 4 are inside the pockets 10a, 10b, moving (rotating) the retainer 9 the circumferential direction was not considered.
In regards to this, the inventors have considered an invention, wherein when placing a ball in a pocket, another balls already installed in a pocket are arranged to press against the edges of the pockets in the circumferential direction so that the retainer rotates in the circumferential direction, therefore of these two kinds of pockets, long and short, it is possible to shorten the long pockets and increase the length in the circumferential direction of the column sections, thus improving the strength and durability of the retainer.
However, if the length of the two kinds of pockets, short and long are unrestricted, it may not be possible to place the balls inside the pockets or maintain the dimensions of the column sections, thus making it impossible to sufficiently improve the durability of the retainer.
In JP Patent Publication KOKAI No. Hei 9-317783, there is a description on the constant velocity joint with eight torque transmitting balls installed therein, specifically on controlling the relation between the pitch circle diameter DP of the eight torque transmitting balls and the torque transmitting ball diameter DB, and the relation between the outer diameter DO of the outer joint member of the constant velocity joint and the pitch circle diameter DS of the tooth profile (female serration) formed in the inner surface of the inner joint member, more specifically, in the relations of 3.3xe2x89xa6DP/DBxe2x89xa65.0 and 2.5xe2x89xa6DO/DSxe2x89xa63.5.
However, a compact constant velocity joint keeping the strength and durability can not be achieved only with the control of the relations between the pitch circle diameter DP and the ball diameter DB and between the outer diameter DO and the pitch circle diameter DS. Particularly, the relations between the pitch circle diameter DS determining the torque capacity of the constant velocity joint, the ball diameter DB greatly affecting the durability of the constant velocity joint, and the ball pitch circle diameter DP and the ball diameter DB must be arranged taking into full consideration the strength of column portions in the retainer or cage.
On the other hand, if the outer diameter of the outer member or ring of the constant velocity joint with six balls installed therein, as conventionally generally constructed, is reduced by 7% by increasing the number of balls to eight, and provided that the ratio of the ball pitch circle diameter DP to the ball diameter DB as in JP Publication KOKAI No. Hei 9-317783 is 5.0, the ball diameter DB would be too small, so that the contact surface pressure between the balls and the inside surfaces of the outer engagement groove and inner engagement groove would be substantially higher comparing with the constant velocity joint with six balls therein, resulting in lower durability. On the contrary, in the case where the ratio of the ball pitch circle diameter DP to the ball diameter DB is 3.3, the length of column portions in the cage is small diameter DB due to the larger ball diameter, so that the strength and durability of the cage is reduced.
In addition, in JP Publication KOKAI No. Hei 9-317783, where the ratio of the outer ring diameter DO to the pitch circle diameter DS is in the range of 2.5 to 3.5, the following points must be further taken into consideration;
where the ball pitch circle diameter DP is set with reference to the inner member or ring and outer member or ring in order to make the constant velocity joint compact, and how the ball diameter DB is set.
For example, if the ball pitch circle diameter DP is larger, the thickness of material of the inner member or ring would be larger to increase the strength and durability of the inner member or ring, and the column portions of the cage or retainer would be longer to increase the strength and durability of the cage or retainer. However, the thickness of material of the outer member or ring would be thin to decrease the strength and durability of the outer member or ring.
On the other hand, if the ball pitch circle diameter DP is smaller, the wall thickness of material of the inner member or ring would be tin to decrease the strength and durability of the inner member or ring, and the column portions of the cage or retainer would be shorter to decrease the strength and durability of the cage or retainer.
The present invention takes the problems mentioned above into consideration, and provides a compact and durable constant velocity joint which makes it possible to increase the lengths of the column sections, improve the strength of the retainer.
Another object of the present invention is to provide a compact and light weight rolling-bearing unit for a vehicle wheel comprising a rolling bearing portion and a constant velocity portion.
Another object of the present invention is provide a compact constant velocity joint the strength and durability in elements of which are substantially the same to the conventional one with six ball installed therein.